The present invention relates to an optical wavelength multiplexer/demultiplexer, and more particularly, relates to a temperature-unreliable, optical wavelength multiplexer/demultiplexer using an arrayed-waveguide diffraction grating (hereinafter, called as xe2x80x9cchannel waveguide arrayxe2x80x9d) composed of a plurality of channel waveguides having a silica glass as a core and having different length from each other, in which loss resulted of grooves that are formed on slab waveguide is reduced and of which spectrum response is optimized, and also relates to a channel waveguide array which is used for a wavelength-division multiplex transmission system.
In the field of optical communication, a wavelength-division multiplexing transmission system that a plurality of signals are put on light having a plurality of wavelengths and the light loaded with the plurality of signals are transmitted through one optical fiber to increase optical communication capacity has been investigated and has been partially implemented in products. In the system, an optical wavelength-division multiplexer/demultiplexer for multiplexing or demultiplexing a plurality of signal lights plays an important role. Among others, an optical wavelength multiplexer/demultiplexer using a channel waveguide array can implement multiplexing/demultiplexing at a narrow wavelength spacing, and hence, can increase the number of multiplexing in communication capacity.
In FIGS. 1 and 2, the optical wave length multiplexer/dimultiplexer comprises a silica substrate 1, a channel waveguide array 3a composed of a plurality of the channel waveguides 3 provided on the substrate 1 in certain pattern, each channel waveguide being composed of cores 2 made of silica glass, and a cladding 4 made of a pure silica glass and provided on the substrate 1 so that the core 2 and the substrate 1 may be covered with the cladding. To the core, titanium oxide (TiO2) is added. The channel waveguide 3 is formed by the core 2 and the cladding 4 and the channel waveguide array 3 has, for example, twenty-two channel waveguides 3. Each channel waveguide 3 of the channel waveguide array 3a has different length from each other so that it becomes longer from one end to the other end (longer side). The channel waveguide array 3a is expected as a key device for the wavelength multiplex transmission system in case that the number of channel is increased, because it can be manufactured by same process and steps regardless of the number of the channel and because deterioration of its characteristics such as loss is less in principle. With respect to a transmitting wavelength, a channel interval and a transmitting center wavelength can be generally set by 100 GHz (approximately, 0.8 nm) or its multiple in accordance with the international standard.
To both sides of the channel waveguide array 3a, a fan-shaped input slab waveguide 5 which may be called, hereinafter, as xe2x80x9cinput waveguidexe2x80x9d and an fan-shaped output slab waveguide 6 which may be called, hereinafter, as xe2x80x9coutput waveguidexe2x80x9d are connected. One input channel waveguide 7 is connected to the fan-shaped input slab waveguide 5 and a plurality of output channel waveguides 8-1xcx9c8-8.
In the above structure, signal lights including various wavelengths input in the input channel waveguide 7 are input through the fan-shaped input slab waveguide 5 in each core 2 of the channel waveguide array 3a. The signal lights input in the channel waveguide array 3a propagate through each core 2 to the fan-shaped output slab waveguide 6, in which a light-collecting position is shifted in the contact surface of the fan-shaped output slab waveguide 6 and the output channel waveguides 8-1xcx9c8-8 because in-phase plane is declined depending on the wavelengths. As a result, the output signal lights in the fan-shaped output slab waveguide 6 are selectively output to the output channel waveguides 8-1xcx9c8-8 in accordance with the shift condition of the in-phase plane, whereby signal lights having different wavelengths are output from the eight waveguides.
A length xe2x80x9cLxe2x80x9d of each channel waveguide 3 in the channel waveguide array 3a changes by thermal expansion and a refractive index of silica glass constituting the core 2 changes with a temperature change. Accordingly, if a temperature changes, for example, from 0xc2x0 C. to 60xc2x0 C., the in-phase plane 9 changes to the in-phase plane 10 as shown in FIG. 1. As a result, a light-collecting position is shifted in accordance with the temperature change, and wavelengths to be demultiplexed change.
In FIG. 3, xe2x80x9cdxe2x80x9d is a pitch of the channel waveguide in the channel waveguide array 3a and xe2x80x9cxcex8xe2x80x9d is an emerging angle of signal light from the channel waveguide to the output slab waveguide 6. If it is required to keep the in-phase plane shown in FIG. 1 to be continuous with respect to certain wavelength, the following equation has to be satisfied.
xe2x80x83(2xcfx80/xcex)Neffxc2x7xcex94L+(2xcfx80/xcex)nsxc2x7d sin xcex8=2 xcfx80mxe2x80x83xe2x80x83(1)
wherein xe2x80x9cxcexxe2x80x9d is a wavelength, xe2x80x9cmxe2x80x9d is the number of degree (m=1, 2, 3, . . . ), xcex94L is a difference of length in the channel waveguide of the channel waveguide array 3a, Neff is an effective refractive index of the channel waveguide array 3a, and ns is an effective refractive index n of the fan-shaped output slab waveguide 6. The effective refractive indexes Neff and ns are equal to the refractive index of silica glass which is used in the channel waveguide array 3a as the core 2, and therefore, n is nearly equal to Neff and ns, respectively. Thus, the following formula can be derived from the formula (1).
xcex94xcex8/xcex94T=(1/n)xc3x97(xcex4n/xcex4T)xc3x97xcex94L/dxe2x80x83xe2x80x83(2)
wherein xe2x80x9cTxe2x80x9d is a temperature, xcex94xcex8 is change of light beam angle (i.e. change of in-phase plane) when change of the temperature is xcex94T, and xcex4n/xcex4T is change of refractive index of the waveguide, and the influences of the thermal expansion are ignored because they are smaller than the change of refractive index. The change of wavelengths to be demultiplexed in accordance with the temperature change is represented by the following formula.                                                                                           Δλ                  /                  Δ                                ⁢                                  xe2x80x83                                ⁢                T                            =                                                (                                      λ                    xc3x97                    n                    xc3x97                                          d                      /                      n                                        ⁢                                          xe2x80x83                                        ⁢                    Δ                    ⁢                                          xe2x80x83                                        ⁢                    L                                    )                                xc3x97                                  (                                                            Δθ                      /                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    T                                    )                                                                                                        =                                                (                                      λ                    /                    n                                    )                                xc3x97                                  (                                      δ                    ⁢                                          xe2x80x83                                        ⁢                                          n                      /                      δ                                        ⁢                                          xe2x80x83                                        ⁢                    T                                    )                                                                                        (        3        )            
For example, a value of xcex94xcex/xcex94T in silica glass to which titanium oxide (TiO2) is added is 0.01 (nm/xc2x0C.), when n≈1.45, xcex4n/xcex4T≈1xc3x9710xe2x88x925, xcex=1550 nm. The optical part materials using such the channel waveguide array 3a are used under the temperature such as 0xc2x0 C. to 60xc2x0 C., in general.
As a result, the channel waveguide array 3a can not be used in practical system, because the wavelengths to be demultiplexed is shifted by 0.6 nm at maximum in case that the temperature changes from 0xc2x0 C. to 60xc2x0 C. In order to reduce the change of center wavelength due to temperature-reliability, it has been proposed that a wedge-shaped groove is provided in a part of the channel waveguide array 3a and an optical resin material is inserted in the groove.
A conventional optical wavelength multiplexer/dimultiplexer in which a wedge-shaped groove having an optical resin material is provided is shown in FIG. 4. A value represented by the formula (2) has to be smaller than a value represented by the formula (3) to reduce the change of center wavelength due to the temperature-reliability. For the purpose, the wedge-shaped groove having a maximum width W is provided in a part of the channel waveguide array 3a and the optical resin material is inserted in the groove. As a result, the shift of demultiplexed wavelengths due to the temperature-reliability in in-phase plane is canceled. This situation is represented by the following formula which is derived from the formula (2).
(xcex4n/xcex4T)xcex94L+(xcex94Lxe2x80x2xc3x97xcex4nxe2x80x2/xcex4T)=0xe2x80x83xe2x80x83(4)
wherein xcex94Lxe2x80x2 is a width difference of each groove in channel waveguide array 3a and the channel waveguide has the width difference xcex94Lxe2x80x2 from its neighboring channel waveguide in this order, nxe2x80x2 is a refractive index of the optical resin, and xcex4nxe2x80x2/xcex4T is a change of refractive index of the optical resin to temperature. For example, in case that a silicon resin is used as the optical resin material, xcex4nxe2x80x2/xcex4T is the following value.
xcex4nxe2x80x2/xcex4T≈xe2x88x9237xc3x9710xe2x88x925[xc2x0C.xe2x88x921]
The change (xcex4nxe2x80x2/xcex4T) of refractive index of the silica glass reliable on temperature is nearly equal 1xc3x9710xe2x88x925 [xc2x0C.xe2x88x921]xe2x80x2 and therefore, xcex94Lxe2x80x2/xcex94L is the following value.
xcex94Lxe2x80x2/xcex94L≈1/37xe2x80x83xe2x80x83(5)
The length difference xcex94L of neighboring channel waveguide is approximately 100 xcexcm and hence, the width difference xcex94Lxe2x80x2 of it is approximately 2 xcexcm. However, the number of channel waveguides in channel waveguide array 3a is 100 to 200 and hence, the maximum width W of the optical resin material 12 is 400 xcexcm.
The length of channel waveguide 3 is designed so that it becomes longer by same length difference xcex94L from inside channel waveguide to outside channel waveguide.
Accordingly, phase change amount "PHgr"xe2x80x2m (xcex) which light wave transmitting through channel waveguide 3 of i-th order from the most inside (shortest) channel waveguide 3 receives can be obtained by the formula based on the channel waveguide 3 existed in the most inside.
xe2x80x83"PHgr"xe2x80x2m(xcex)=2xcfx80naxc2x7ixcex94L/xcexxe2x80x83xe2x80x83(6)
wherein xcex is a wavelength of light wave in vacuum and na is effective refractive index of channel waveguide 3.
According to the formula, the effective refractive index of light wave in the vicinity of the connecting face of the channel waveguide 3 and the output slab waveguide 6 declines depending on the wavelength, and the light wave subjected to the phase change by each channel waveguide 3 is interfered in the output slab waveguide 6 and the interfered wave is output from the output channel waveguide 7.
In case that wavelength changes, the light-collecting position is shifted in the connecting face of the output slab waveguide 6 and the output channel waveguide 8, because a direction of the in-phase plane is different by wavelength. Accordingly, the light wave having different wavelength can be taken from each output channel waveguide, whereby light multiplex/demultiplex can be realized. The wavelength xcex emerged from the output channel waveguide 8 provided on a symmetrical axis 11 of the output slab waveguide 6 is represented by the formula.
xcex=na xcex94L/mxe2x80x83xe2x80x83(7)
wherein xe2x80x9cmxe2x80x9d is a diffraction degree.
In case that a light circuit is constructed by a normal material, a refractive index of the material changes by thermo-optical effects due to temperature change to change na, and a length of the channel waveguide 3 changes by a thermal expansion to change xcex94L. Accordingly, the in-phase plane of light wave in the vicinity of the connecting face of the channel waveguide 3 and the output slab waveguide 6 declines depending on the temperature to change the output wavelength.
On the other hand, in case that temperature changes by xcex94T in the light wave which is output from the output channel waveguide provided on a symmetrical axis 11 of the output slab waveguide 6, the wavelength change amount xcex94xcex is calculated by the formula (8) which is obtained by differentiating the formula (7) by T.                                                         Δλ              =                              Δ                ⁢                                  xe2x80x83                                ⁢                                  L                  /                                      m                    ⁡                                          [                                                                                                    ⅆ                                                          n                              a                                                                                /                                                      ⅆ                            T                                                                          +                                                                                                            n                              a                                                        ·                                                          1                              /                              Δ                                                                                ⁢                                                      xe2x80x83                                                    ⁢                                                      L                            ·                                                                                          ⅆ                                                                  (                                                                      Δ                                    ⁢                                                                          xe2x80x83                                                                        ⁢                                    L                                                                    )                                                                                            /                                                              ⅆ                                T                                                                                                                                                        ]                                                                      ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                                                                                        =                                                λ                  /                                                            n                      a                                        ⁡                                          [                                                                                                    ⅆ                                                          n                              a                                                                                /                                                      ⅆ                            T                                                                          +                                                                                                            n                              a                                                        ·                                                          1                              /                              Δ                                                                                ⁢                                                      xe2x80x83                                                    ⁢                                                      L                            ·                                                                                          ⅆ                                                                  (                                                                      Δ                                    ⁢                                                                          xe2x80x83                                                                        ⁢                                    L                                                                    )                                                                                            /                                                              ⅆ                                T                                                                                                                                                        ]                                                                      ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                                                                        (        8        )            
If the light circuit is constituted by a silica material and dna/dT is equal to a temperature coefficient of the silica material in the formula (8), dna/dT is nearly 1xc3x9710xe2x88x925 [xc2x0C.xe2x88x921], 1/xcex94Lxc2x7d(xcex94L)/dT is nearly 5xc3x9710xe2x88x927 and na is nearly 1.45, and hence, xcex94xcex/xcex94T is nearly equal to 0.01 (nm/xc2x0C.) when xcex is 1550 (nm). Therefore, if the optical wavelength multiplexer/demultiplexer is used at 0xc2x0 C. to 60xc2x0 C., its wavelength is shifted by 0.6 nm at maximum. It is impossible to use the multiplexer/demultiplexer as practical system under such wavelength sift, and it is necessary to control the temperature of light circuit in order to resolve the problems.
An electric power has to be supplied to the channel waveguide array 3a using a silica material which has a temperature coefficient of approximately 0.01 (nm/xc2x0C.), because an active control system that the center wavelength is set by using a heater (not shown) or a Peltier element (not shown) is applied. Such system requires expensive cost. An optical wavelength multiplexer/demultiplexer using temperature-unreliable, channel waveguide array has been investigated (Inoue et al., Shingakukai Sogo Taikai C-3-117 (1998)). According to the investigation, a groove (not shown) is formed on the channel waveguide array 3a by etching, and a resin material having a temperature coefficient opposite to that of silica is inserted in the groove to make the transmitting wavelength to be temperature-unreliable. Further, according to it, the center wavelength is not precisely adjusted by using a heater or Peltier element because a waveguide element is temperature-unreliable. Therefore, the input optical fiber 7a is directly connected to the end 25 of the input slab waveguide 5 as shown in FIG. 5, and the center wavelength is controlled by adjusting the connecting position of the fiber 7a. 
An example of conventional optical wavelength multiplexer/demultiplexer that the wavelength shift is reduced and the temperature control is omitted is shown in FIG. 4. A groove 12 is provided on a part of the channel waveguide 3 and the material having different temperature coefficient of refractive index from that of light circuit is filled in the groove to cancel a decline of in-phase plane due to temperature. This is written in Y. Inoue et al. xe2x80x9cA thermal silica-based arrayed-waveguide grating (AWG) multiplexerxe2x80x9d ECOC 97 technical digest, pp. 33 to 36, 1997. However, according to the method, it is the problem that diffraction loss becomes more by the groove 12, because the groove 12 is provided in the way of the channel waveguide 3 having a two-dimensionally light-enclosing effect.
The optical wavelength multiplxer/demultiplexer is shown in FIG. 6, which is proposed to reduce the diffraction loss in the groove. The groove 20 is provided on the input slab waveguide 5 or the output slab waveguide 6 and the material having different temperature coefficient of refractive index from that of light circuit is filled in the groove. There are advantages that increase of the diffraction loss can be controlled, because it is one dimensional light that is shut in the slab waveguide 5, 6.
In a conventional optical multiplexer/demultiplexer, if it is constructed by several hundreds of waveguides, the maximum width W of the wedge-shape groove reaches several hundreds xcexcm and thus, diffraction loss is increased and an additive loss of approximately 4 to 6 dB is generated. And, a groove width of not more than sub-micron meter, which is smoothly changed with high accuracy is required in order to obtain good demultiplex characteristics having small cross-talk. However, if the groove is provided in such wide range as conventional one, cross-talk is deteriorated because it is difficult to increase production accuracy of the groove.
According to another conventional optical multiplexer/demultiplexer having the grooves 20 in the input slab waveguide 5 as shown in FIG. 6 and the channel waveguide array 3a as shown in FIG. 9, increase of the diffraction loss is not sufficiently protected because a width of the grooves in the input slab waveguide 5 has to be substantially same as those in the channel waveguide array 3a. Accordingly, it is difficult to constitute optical multiplexer/demultiplexer which is practically durable, and the problems in spectrum response of light wave occur. The degree of influence to the loss by the grooves 20 is shown in FIG. 7 (without groove 20) and FIG. 8 (with groove 20) in which design value is obtained by a beam propagation method. The minimum loss in the design value is set so that it is consistent with that in the measured value. In FIG. 7 (without groove 20), the graph shape of the passing region (main loop) A in the design value is substantially same as that in the measured value, and the minimum loss is 4.3 dB. On the other hand, in FIG. 8 (with groove 20), the graph shape of the passing region in the design value is broader than that in the measured value, and the minimum loss is 7.1 dB.
The problem of the spectrum response is illustrated in FIG. 9. An aberration occurs between the light wave emanating from the point xe2x80x9cOxe2x80x9d corresponding to the exit of the input channel waveguide 7 and propagating in the vicinity of the center of the groove 20, and the light wave emanating from the point xe2x80x9cOxe2x80x9d corresponding to the exit of the input channel waveguide 7 and propagating apart from the center of the groove 20, because the refractive angles xcex81 and xcex82 are different from each other. Accordingly, if the light wave emanates with keeping the aberration, the spectrum response of the light wave in the optical multiplexer/demultiplexer with the grooves provides more discrepancy than that in the optical multiplexer/demultiplexer with no groove.
According to the conventional optical wavelength multiplexer/demultiplexer with the grooves 20 on the slab waveguide 5 or 6 as the above illustration, it is difficult to keep the loss due to the groove 20 lower and problems exist in the spectrum response, and therefore, it is difficult to make practical use of it at present.
Further, if the input optical fiber is directly connected to the end surface of the slab waveguide, there is a problem that it is hard to optimize distribution of electric fields to provide wide and flat band width or low cross-talk by introducing a Y-branch type waveguide, a parabolic hone type waveguide or a taper type waveguide in front of the slab waveguide. And, when the optical fiber is set on the end surface of the slab waveguide, there is another problem that the discrepancy of the axis has to be within the range of sub xcexcm and several xcexcm in order to accurately set the center wavelength.
A first object of the invention is to provide an optical wavelength multiplexer/demultiplexer with a reduced additive loss and less deteriorated cross-talk.
A second object of the invention is to provide an optical wavelength multiplexer/demultiplexer with less increase of loss caused by grooves and with optimized spectrum response.
A third object of the invention is to provide a temperature-unreliable, optical wavelength multiplexer/demultiplexer capable of finely adjusting set of the center wavelength and optimizing electric field distribution of the signal lights in the channel waveguide array to expand a band width and to provide a flatter loss or to reduce a cross-talk.
A fourth object of the invention is to provide an optical wavelength multiplexer/demultiplexer capable of providing a flatter loss in a wider band width and accurately setting the center wavelength, and having low cross-talk.
The objects of the invention can be attained in accordance with the each feature of the present invention.
According to the feature of the invention, is an optical wavelength multiplexer/demultiplexer includes an optical wavelength multiplexer/demultiplexer including a substrate; an input channel waveguide provided on the substrate; an input slab waveguide of which one end is connected to the input channel waveguide; a channel waveguide array of which one side is connected to the other side of the input slab waveguide and which has a plurality of channel waveguides, each of the plurality of channel waveguides differing in length from its neighboring waveguide by a predetermined amount; an output slab waveguide of which one side is connected to the other side of the channel waveguide array; and a plurality of output channel waveguides which are connected to the other side of the output slab waveguide;
wherein said input slab waveguide or output slab waveguide having one of a temperature compensation material, in its light path, having an opposite sign of refractive index-temperature change to the plurality of channel waveguides; a material capable of canceling change in in-phase plane of light having each wavelength which occurs in the vicinity of the channel waveguide array and the slab waveguide, the material being provided in the curved form so that it may cross the light traveling direction, and a waveguide element for band width adjustment on which a waveguide to adjust band width of wavelength multiplexing light is provided.
The thickness of the temperature compensation material provided on the input slab waveguide or output slab waveguide is thinner than the thickness of the temperature compensation material provided on the channel waveguide array. As a result, the additive loss and cross-talk are reduced.
A preferred embodiment in the feature of the invention is that the temperature compensation material is a wedge-shaped multi-component glass material is provided in the light path of the input slab waveguide or output slab waveguide. Another preferred embodiment of the invention is that a wedge-shaped optical resin is provided in the light path of the input slab waveguide or output slab waveguide.
A preferred embodiment of the invention is that the input slab waveguide or output slab waveguide is composed of a curved groove crossing the light traveling direction, and a filler which is filled in the curved groove and which has a temperature incline of refractive index different from that of the materials making up the input slab waveguide or output slab waveguide including the curved groove.
It is preferred in the structure that the curved groove is provided so that a center of curvature in the wall surface of the groove exists in the vicinity of the face between the input slab waveguide and the input channel waveguide, or between the output slab waveguide and the output channel waveguide. It is preferred that the number of the curved grooves is a plurality, and that the curved grooves are disposed in the light traveling direction. It is preferred that a width of the groove is smaller and a plurality of the grooves are provided, because increase of the loss is restrained.
As the examples of the material capable of canceling the change in in-phase plane of each wavelength light caused by temperature change and the filler which is filled in the curved groove, there are an optical resin including a silicon resin, an epoxy resin and a polymethyl methacrylate resin, or a multi-component glass material including sodium, potassium and calcium. As the examples of the material constituting the input and output slab waveguides, the input and output channel waveguides and the channel waveguides of the channel waveguide array is a silica material. In case that the slab waveguide made of the optical resin and the silica material is combined, particularly marked advantages can be obtained because the optical resin provides a negative refractive index-temperature change and the silica material provides a positive refractive index-temperature change, whereby the optical wavelength multiplexer/demultiplexer becomes temperature-unreliable.
By the waveguide element for band width adjustment, set of the center wavelength is finely adjusted, electric field distribution of the signal light in the channel waveguide array is optimized, a band width is expanded, a flatter loss is provided and a cross-talk is reduced. The waveguide element for band width adjustment can be provided with the waveguide of which one end is expanded in the taper-shaped or in the parabolic hone-shaped toward light emanating side. In the structure, it is also possible that the input slab waveguide is divided into twoparts, one of the divided parts existing in the entrance side thereof is provided with the waveguide element for band width adjustment, and the other of the divided parts existing in the emanating side thereof is provided on the substrate. And, the waveguide element for band width adjustment can be provided with a slit which exists in the waveguide thereof and is expanded in the entrance and emanating direction.
In the structure, the optical wavelength multiplexer/demultiplexer can be provided with a plurality of additive waveguides in the vicinity of the waveguide element for band width adjustment and the output waveguides. It is preferred that each of the waveguide is temperature-unreliable. According to the structure, a position and a shape of the waveguide element for band width adjustment are optimized by providing it between the input optical fiber and the input slab waveguide, whereby adjustment of wave shape (e.g adjustment to wider band width) can be implemented. Further, by providing the additive waveguides between the waveguide for band width adjustment and the output waveguide, the center wavelength can be accurately set, even if the axis of the element is shifted when the element is fixed.